You have now seen several examples of curly arrows representing the movement of electrons during a reaction, and it is time to discuss them in detail. It is no exaggeration to say that this simple device is the one most powerful tool chemists have for explaining simply and accurately how reactions work—in other words the mechanisms of reactions. Curly arrows are to reactions what structural diagrams are to molecules. We discussed the guidelines for drawing structures in Chapter 2, explaining that although the structure of a molecule may be very complex, a good structural diagram will represent all of its important features without unnecessary detail. Curly arrows are similar: you have seen how reactions involve the overlap and summation of molecular orbitals to make new molecular orbitals, and the movement of electrons within those orbit als. Curly arrows allow us to represent all the important features of those interactions and electron movements very simply, without being concerned with unnecessary detail. It’s now time to outline some guidelines for writing mechanisms with curly arrows.